The present invention relates to a radio signal receiving system having a plurality of receiving sections and, more particularly to an improved signal combiner for such a system.
Such a receiving system comprises a plurality of receiving sections. A detector detects whether or not the outputs of the receiving sections are in normal state. A signal combiner operates responsive to the output of the detector for combining the outputs of the receiving sections and/or selectively allowing one of the outputs to pass therethrough. This construction ensures a considerably high reliability, for the receiving system.
In order to either combine the outputs of the receiving sections or select one of them, a conventional signal combiner includes detector-responsive switches which are respectively connected into the signal paths between the receiving sections and the combining circuit placed in the signal combiner. Two receiving sections are usually employed, with two signal paths coupled thereto. When both of the signal paths are in a normal condition, the signal on one transmission path is combined with the signal on the other path, by means of a resistor hybrid circuit disposed at a stage positioned after the switches. The resulting combined signal is led to an output terminal.
When either of the transmission paths is driven out of the normal state, only the signal transmitted through the normal path is led to the output terminal, through the hybrid circuit. In such a signal combiner operation, the switches are connected in series, with respect to the signal paths. For this reason, the output impedance and the output voltage of the combiner may vary depending upon whether the output is a combined signal or the switch-selected signal.
An object of the present invention is, therefore, to provide a signal combiner which is free from the problems involved in the conventional circuit.
According to the present invention, a radio signal receiving system has first and second signal receiving sections and a signal combiner for selectively passing the outputs of the signal receiving sections, in order to provide a combined signal. The signal combiner has first and second input terminals for respectively receiving the outputs of the first and second receiving sections. A combining circuit has first and second arms and an output terminal. First and second switches are connected between the first input terminal and the first arm, and between the second input terminal and the second arm, respectively a third switch is connected between the first and second arms and is interlocked with the first and second switches. Means are connected to the first and second input terminals for controlling the first, second and third switches so that, when the two outputs derived from the first and second receiving sections are in normal state, the first and second switches are closed, with the third switch opened. When either of the two outputs is driven out of the normal state, the first or second switch receiving the normal signal and the third switches are closed, with the first or second switch receiving the abnormal signal opened.